Fuel cells are electrochemical devices which can convert chemical energy stored in fuels to electrical energy with high efficiencies. They comprise an electrolyte between electrodes. Solid oxide fuel cells (SOFCs) are characterized by the use of a solid oxide as the electrolyte.
In solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow can be a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol, or methanol. The fuel cell, operating at a typical temperature between 650° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
In recent years considerable interest has been shown towards the development of SOFC electrolyte compositions of high ionic conductivity. Doped cerium oxide, lanthanum gallate and zirconium oxide are the most suitable candidates. However, in order to achieve sufficient ionic conductivity high operation temperature is often required, deteriorating the life of the fuel cell components and requiring the use of expensive materials in the fuel cell stack, such as chromium alloy interconnects. Therefore, it is highly desirable to lower the operation temperature of SOFCs and one important step to achieve this is the development of an electrolyte composition with higher ionic conductivity than yttria-stabilized zirconia (YSZ), the state of the art SOFC electrolyte material.
Doping zirconia with aliovalent dopants stabilizes the high temperature cubic fluorite phase at room temperature leading to an increase in oxygen vacancy concentration, oxygen mobility and ionic conductivity. Complex studies have demonstrated a correlation between the dopant and host ionic radii and the existence of a critical dopant cation radius that can ensure maximum conductivity. It has been suggested that a good evaluation of the relative ion mismatch between dopant and host would be to compare the cubic lattice parameter of the host oxide and the pseudocubic lattice parameter of the dopant oxide, a smaller size mismatch being preferred for obtaining high ionic conductivity.
Numerous attempts to find the appropriate dopant for stabilizing the cubic phase have been made (Y, Yb, Ce, Bi, etc). Among these, scandia stabilized zirconia shows the highest ionic conductivity with Sc3+ concentration of 11 mole % (2-3 higher than YSZ at 800° C.) due to a lower activation energy than YSZ. The complex phase diagram in the Sc2O3—ZrO2 system is still under debate, with several phases identified in the dopant rich segment of the phase diagram, as monoclinic, tetragonal and rhombohedral intermediate phases appear at low temperatures. One example is the distorted rhombohedral β phase (Sc2Zr7O17), that undergoes a rhombohedral-cubic phase transition around 600-700° C. and induces a steep decrease in conductivity in this temperature region. This transition is not favorable from the point of view of thermal expansion mismatch and may be related to order-disorder transition of oxygen vacancies.
Co-doping in zirconia systems may produce cheaper, stable compositions with enhanced ionic conductivity. Co-dopants in the scandia zirconia system include Ce, Y, Yb and Ti, the former (1 mole % CeO2) being the most successful to date in stabilizing the cubic phase at room temperature and very high conductivity values have been measured by Lee et al, 135 mS/cm at 800° C. in air (SSI 176 (1-2) 33-3 (2005)). However, concerns about long term stability, especially at the interface with the fuel electrode, due to Ce3+ presence, have been reported.
In general, CaO or MgO are used for improving the toughness of zirconia ceramics by stabilizing the tetragonal phase at room temperature.
However, previous studies for doping zirconia with alkaline earth metal cations alone have been unsuccessful in improving the conductivity because of a high tendency of defect association and a lower thermodynamic stability of cubic fluorite ZrO2—CaO and ZrO2—MgO solid solutions.